The overall cellular response to ionizing radiation exposure is not limited to cells directly irradiated, but includes neighboring bystanders. Essentially every endpoint used routinely to test for changes in cells directly exposed to radiation, such as mutation, apoptosis, micronuclei formation or cell cycle checkpoints, has been studied in bystanders. This phenomenon has caused a paradigm shift in thinking about biological effects and associated health risks of radiafion exposure; the impacted cell population is larger than once believed. The mechanism regulating bystander effects is not well understood. However, certain gap junction proteins, i.e., Connexin 32, and proteins like Cox-2 control the overall process, whereas p21 just regulates the radiation-induced Gl to S checkpoint. We have shown that Rad9 regulates the bystander response to radiation, since Rad9 null enhances bystander apoptosis and micronuclei formation. Rad9 protein, which controls cell cycle checkpoints, DNA repair, apoptosis and transactivation of downstream target genes in directly irradiated cells, likely impacts on the bystander response via more than one pathway. In collaboration with investigators in Projects 2 and 3, we show that Rad9 is essential for bystander induction of Cox-2 and p21 (Project 2), and production of Connexin 32 in HeLa cells, which are normally devoid of the protein, increases bystander radiation resistance and levels of Rad9 as well as TCTP (Project 3), which both function in the bystander process. We also show that TCTP and Rad9 physically interact (Project 3). Our overarching hypothesis is that Rad9 plays an important role in radiation-induced bystander effects via mechanisms involving at least Cox-2, p2I, Connexin 32 and TCTP. Specific aims to address this hypothesis are: I. Determine the activity of Rad9 in nuclear versus cytoplasmic irradiation-induced bystander effects; 2. Analyze Rad9 functional domains and protein-protein interactions for roles in the bystander process; 3. Establish the significance of Rad9-dependent bystander induction of Cox-2, p21 and other genes for bystander regulation; 4. Determine if Rad9 is critical for bystander mutagenesis; and 5. Define the significance of junctional communication on the regulation and function of Rad9. These studies will reveal mechanisms by which Rad9 regulates radiation-induced bystander effects, and will impact on understanding the biological response to radiation exposure, with implications for improving radiotherapy and better defining health risk.